1. Field of the Invention
The present invention relates to a superconducting quantum interference device (SQUID), and more particularly, to a method of controlling the characteristics of a double relaxation oscillation SQUID having a reference junction.
2. Background of the Related Art
In general, a SQUID refers to a high-sensitivity magnetometer capable of detecting up to a minute magnetic flux density employing Josephson effect. More particularly, a double relaxation oscillation SQUID (hereinafter, referred to as a “DROS”) has a great magnetic flux-voltage characteristic and a high transfer coefficient and can detect a fast magnetic flux signal using a simple pre-amplifier.
FIG. 1 is an equivalent circuit diagram of a DROS having a reference SQUID (RS-DROS) in the related art. FIG. 2 is an equivalent circuit diagram of a DROS having a reference junction (RJ-DROS) in the related art.
As shown in FIG. 1, in the reference SQUID type DROS (RS-DROS) 11 having a signal SQUID 12 and a reference SQUID 13, current flows from a reference voltage application current source current controller 18 to a reference SQUID coil 16 and changes the magnetic flux of the reference SQUID coil 16. Accordingly, a critical current of the reference SQUID 13 is controlled to control a magnetic flux-voltage characteristic so that the DROS operates under an optimal condition.
An external magnetic flux reacts in an input coil Mi 14 of the signal SQUID 12. A DROS output at both ends of the reference SQUID 13 is detected by a pre-amplifier 19. A constant current source Ib for operating the DROS is controlled by an operation application current source current controller 17 at normal temperature. A feedback coil Mf 15 constitutes a flux-locked loop (FLL) and is used to measure an external magnetic flux. A RS-DROS 11 is located at 4.2 K. The two current controllers (the operation application current source current controller 17 and the reference voltage application current source current controller 18) and the pre-amplifier 19 operates at normal temperature.
Meanwhile, as shown in FIG. 2, the reference junction-type DROS (RJ-DROS) 21 having a signal SQUID 22 and a reference junction 23 has the reference junction, instead of the reference SQUID, and a fixed reference current. The RJ-DROS 21 has a simple structure and is easy to operate.
A current Ib for operating the RJ-DROS is controlled by an operation application current source current controller 26 at normal temperature. An external magnetic flux reacts through an input coil 24 of the signal SQUID 22. An output of the RJ-DROS at both sides of the reference junction 23 is detected by a pre-amplifier 27. A feedback coil 25 is used to operate the DROS in FLL mode.
However, in the related art DROS, i.e., the reference SQUID type DROS 11 having the signal SQUID 12 and the reference SQUID 13, two SQUID must be formed at the same time and the magnetic flux coil 16 is required in order to control the reference current of the reference SQUID 13. Accordingly, the related art DROS is complicated in structure and is difficult to operate.
To solve the problems, the reference junction-type DROS 21 having the signal SQUID 22 and the reference junction 23 was developed in order to simplify the structure and manufacture. However, the reference junction-type DROS 21 is problematic in that characteristics of the DROS cannot be controlled because the reference current of the reference junction 23 is fixed.